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Propulsion System

Picture of the Saturn 1B rocket and a nozzle test.
On this page, we show a picture of a Saturn 1B at the left and a picture of a rocket engine test at the right. For the picture at the right, we only see the outside of the rocket nozzle, with the hot gas exiting out the bottom. The first stage of the Saturn 1B was powered by eight liquid rocket engines burning a hydrocarbon fuel with liquid oxygen. The second stage used a single liquid hydrogen and liquid oxygen powered engine and was used to place the Apollo spacecraft into low Earth orbit.

Propulsion System

There are four major components to any full-scale rocket: the structural system, or frame, the payload system, the guidance system, and the propulsion system. The propulsion of a rocket includes all of the parts which make up the rocket engine, the tanks pumps, propellants, power head and rocket nozzle. The function of the propulsion system is to produce thrust.

Thrust is the force which moves a rocket through the air and through space. Thrust is generated by the propulsion system of the rocket. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton’s third law of motion. For every action there is an equal and opposite reaction. In any propulsion system, a working fluid is accelerated by the system and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas.

Rocket Engine

In a rocket engine, fuel and a source of oxygen, called an oxidizer, are mixed and exploded in a combustion chamber. The combustion produces hot exhaust which is passed through a nozzle to accelerate the flow and produce thrust. For a rocket, the accelerated gas, or working fluid, is the hot exhaust produced during combustion. This is a different working fluid than you find in a gas turbine engine or propeller powered aircraft. Turbine engines and propellers use air from the atmosphere as the working fluid, but rockets use the combustion exhaust gases. In outer space there is no atmosphere so turbines and propellers cannot work there. This explains why a rocket works in space but a turbine engine or a propeller does not work.

Liquid & Solid Rockets

There are two main categories of rocket engines: liquid rockets and solid rockets. In a liquid rocket, the propellants, the fuel and the oxidizer, are stored separately as liquids and are pumped into the combustion chamber of the nozzle where burning occurs. In a solid rocket, the propellants are mixed together and packed into a solid cylinder. Under normal temperature conditions, the propellants do not burn; but they will burn when exposed to a source of heat provided by an igniter. Once the burning starts, it proceeds until all the propellant is exhausted. With a liquid rocket, you can stop the thrust by turning off the flow of propellants; but with a solid rocket, you have to destroy the casing to stop the engine. Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks. The propellants are loaded into the rocket just before launch. A solid rocket is much easier to handle and can sit for years before firing.

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